Silicon semiconductor substrates, which are used to manufacture integrated circuits, are susceptible to contamination from the diffusion of impurities. The contaminants may come from a surface material upon which the wafer is placed for processing or from any of the many processes to which the wafer is subjected. The contaminants may be picked up from the backside or the frontside of the wafer and may diffuse therethrough in either direction. The contaminants, which typically are heavy metals such as gold, copper, iron, nickel, etc., may show up on a surface of the wafer as a haze which is visible to the human eye.
Contaminants may, however, be both beneficial and detrimental to the integrated circuits being formed on the semiconductor wafer. Contaminants may cause low lifetime or high leakage current. Thus contaminants are not desirable wherever high lifetime is sought, such as in memory-type semiconductor devices. Whenever high lifetime is desired, contaminants must be gettered away from the device by some type of gettering method. Conversely, for example, in high speed switching devices, low lifetime is desired and therefore, contaminants do not need to be gettered.
Typical gettering methods currently in use include backside gettering, intrinsic oxygen precipitation gettering or a combination of backside and intrinsic oxygen precipitation gettering. In backside gettering, mechanical abrasions, silicon nitride, high phosphorus concentration, or a polysilicon layer is applied to the backside of the semiconductor wafer. These materials serve as an effective sink for contaminants in the wafer which are diffused through the thickness thereof. In oxygen precipitation gettering, the wafer is processed through a high temperature-low temperature-high temperature cycle to produce denuded zones near the top and bottom surfaces of the wafer which are relatively clear of contaminants, oxygen precipitates and other defects congregate in the center portion of the wafer and act as gettering sites.
The trend in semiconductor processing is toward lower processing temperatures and layer diameter, thus thicker wafers. Unfortunately, the thicker a substrate becomes, the less effective backside gettering becomes. Likewise, intrinsic oxygen precipitation gettering becomes less effective with lower processing temperatures. Thus, there is a need for a method to effectively getter a relatively thick substrate at relatively lower processing temperatures, as well as to discretionarily getter devices formed thereon.